US20230050795A1

US20230050795A1 - Speech recognition apparatus, method and program

Info

Publication number: US20230050795A1
Application number: US17/793,000
Authority: US
Inventors: Takafumi MORIYA; Yusuke Shinohara
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2020-01-16
Filing date: 2020-01-16
Publication date: 2023-02-16
Also published as: JPWO2021144901A1; WO2021144901A1; JP7294458B2

Abstract

A score integration unit 7 obtains a new score Score (l_1:n ^{b, c}) that integrates a score Score (l_1:n ^{b, c}) and a score Score (w_1:o ^{b, c}). This new score Score (l_1:n ^{b, c}) becomes a score Score (l_1:n ^b) in a hypothesis selection unit 8. Thus, the score Score (l_1:n ^b) can be said to take into account the score Score (w_1:o ^{b, c}). In a speech recognition apparatus, first information is extracted on the basis of the score Score (l_1:n ^b) taking into account the score Score (w_1:o ^{b, c}). Thus, speech recognition with higher performance than that in the related art can be achieved.

Description

TECHNICAL FIELD

The present disclosure relates to a speech recognition technology.

BACKGROUND ART

In speech recognition systems using neural networks in recent years, it is possible to output a word sequence directly from a speech feature. As a learning method for a speech recognition system that outputs a word sequence directly from this acoustic feature, for example, a technique described in NPL 1 is known.
In the technique stated in NPL 1, conversion processing of “acoustic feature⇒phonemic sequence” is performed as processing in the previous stage, and conversion processing of “phonemic sequence⇒word sequence” is performed as processing in the subsequent stage.

CITATION LIST

Non Patent Literature

NPL 1: Shiyu Zhou et. al, “Syllable-based Sequence-to-sequence Speech Recognition with the Transformer in Mandarin Chinese,” INTERSPEECH, pp.791-795, 2018

SUMMARY OF THE INVENTION

Technical Problem

In the technique stated in NPL 1, the conversion processing of the “acoustic feature⇒phonemic sequence” in the previous stage and the conversion processing of the “phonemic sequence⇒word sequence” in the subsequent stage are performed independently. In other words, in the conversion processing of the “acoustic feature⇒phonemic sequence” in the previous stage, the conversion processing of the “phonemic sequence⇒word sequence” in the subsequent stage is not considered.
An object of the present disclosure is to provide a speech recognition apparatus, a method, and a program with higher speech recognition performance than that in the related-art.

Means for Solving the Problem

In a speech recognition apparatus according to an aspect of the present disclosure, B and C are predetermined positive integers, b=1, . . . , B and c=1, . . . , C hold, and a hypothesis HypSet(b) includes a first information sequence l_1:n−1 ^bfrom an index 1 to an index n−1 immediately before index n that is currently being processed, and a score Score (l_1:n−1 ^b) representing a likelihood of the first information sequence l_1:n−1 ^b. The speech recognition apparatus includes: an intermediate feature calculation unit configured to input an input acoustic feature in a predetermined neural network and calculate an intermediate feature; a character feature calculation unit configured to calculate a character feature L_n−1 ^bcorresponding to first information l_n−1 ^bof the index n−1 in a hypothesis b; an output probability distribution calculation unit configured to calculate, using the intermediate feature and the character feature L_n−1 ^b, an output probability distribution Y_n ^bin which a plurality of output probabilities corresponding to respective pieces of the first information are arranged; a first information extraction unit configured to extract first information l_n ^{b, c}having a c-th highest output probability among the output probability distributions Y_n ^b, and a score Score (l_n ^{b, c}) that is an output probability corresponding to the first information l_n ^{b, c}; a hypothesis creation unit configured to create a first information sequence l_1:n ^{b, c}coupling the first information sequence l_1:n−1 ^band the first information l_n ^{b, c}, and a score Score (l_1:n ^{b, c}) representing a likelihood of the first information sequence l_1:n ^{b, c}, a first conversion unit configured to convert the first information sequence l_1:n ^{b, c}into a second information sequence w_1:o ^{b, c}using a predetermined model, and obtain a score Score (w_1:o ^{b, c}) representing a likelihood of the second information sequence w_1:o ^{b, c}; a score integration unit configured to obtain a new score Score (l_1:n ^{b, c}) that integrates the score Score (l_1:n ^{b, c}) and the score Score (w_1:o ^{b, c}); a hypothesis selection unit configured to select B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c}), and generate a new hypothesis including a plurality of new scores selected and a first information sequence corresponding to the plurality of new scores to set new hypotheses HypSet(1), . . . , HypSet(b) to be used at an index n+1 that is one after the index n that is currently being processed; a control unit configured to repeat processing of the intermediate feature calculation unit, the character feature calculation unit, the output probability distribution calculation unit, the first information extraction unit, the hypothesis creation unit, the first conversion unit, the score integration unit, and the hypothesis selection unit, until a predetermined end condition is satisfied; and a second conversion unit configured to, when the predetermined end condition is satisfied, convert at least a first information sequence l_k:n ¹corresponding to a score Score (l_1:n ¹) having a highest value into a second information sequence w_1:o ¹, using a predetermined model.

Effects of the Invention

By taking into account conversion processing of “first information sequence⇒second information sequence” in a subsequent stage in conversion processing of “acoustic feature⇒first information sequence” in a previous stage, speech recognition with higher performance than that in the related-art can be achieved. More particularly, extraction of first information is performed on the basis of a new score Score (l_1:n ^b) considering a score Score (w_1:o ^{b, c}), speech recognition with higher performance than that in the related art can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a functional configuration of a speech recognition apparatus.

FIG. 2 is a diagram illustrating an example of a processing procedure of a speech recognition method.

FIG. 3 is a diagram illustrating a functional configuration example of a computer.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of a speech recognition apparatus and a speech recognition method will be described with reference to the drawings.
Speech Recognition Apparatus and Speech Recognition Method
As illustrated in FIG. 1 , the speech recognition apparatus includes, for example, an intermediate feature calculation unit 1, a character feature calculation unit 2, an output probability distribution calculation unit 3, a first information extraction unit 4, a hypothesis creation unit 5, a first conversion unit 6, a score integration unit 7, a hypothesis selection unit 8, a control unit 9, and a second conversion unit 10.
The speech recognition method is achieved, for example, by each component of the speech recognition apparatus performing processing of steps S1 to 10 described below and illustrated in FIG. 2 .
Hereinafter, each component of the speech recognition apparatus will be described.
Intermediate Feature Calculation Unit 1
An acoustic feature X is input to the intermediate feature calculation unit 1.
The intermediate feature calculation unit 1 calculates an intermediate feature H by inputting the input acoustic feature X to a predetermined neural network (step S1).
The calculated intermediate feature H corresponding to each piece of the first information is output to the output probability distribution calculation unit 3.
In the following description, information expressed in a first expression format is used as first information, and information expressed in a second expression format is used as second information.
An example of the first information includes a phoneme or a grapheme. An example of the second information includes a word. Here, the words are expressed by alphabetical letters, numbers, symbols in a case of English, and are expressed by hiragana, katakana, kanji, alphabets, numbers, symbols in a case of Japanese. The language corresponding to the first information and the second information may be languages other than English and Japanese.
The first information is a kana sequence, and the second information may be a kana-kanji mixture sequence.
The predetermined neural network is a multi-stage neural network.
The intermediate feature is defined by Equation (1) of Reference 1, for example. Reference 1: G. Hinton, L. Deng, Dong Yu, George E. Dahl, Abdel-rahman Mohamed, Navdeep Jaitly, Andrew Senior, Vincent Vanhoucke, Patric Nguyen, Tara N. Sainath, and Brain Kingsbury, “Deep neural networks for acoustic modeling in speech recognition,” IEEE Signal Processing Magazine, Vol. 29, No. 6, pp. 82-97, 2012.
In general, the main stream for speech recognition is to recognize candidates for various hypotheses while leaving the candidates by the number B of beam widths. Thus, assuming b=1, . . . , B, processing from step S2 to step S7 described below is performed for each b. B is a predetermined positive number.
Character Feature Calculation Unit 2
First information l_n−1 ^bof an index n−1 in a hypothesis b is input to the character feature calculation unit 2.
The character feature calculation unit 2 calculates a character feature L_n−1 ^bcorresponding to the first information l_n−1 ^bof the index n−1 in the hypothesis b (step S2).
The calculated character feature L_n−1 ^bis output to the output probability distribution calculation unit 3.
When the first information l_n−1 ^bis expressed by a vector such as a one-hot vector, the character feature calculation unit 2 calculates the character feature L_n−1 ^bby, for example, multiplying a vector corresponding to the first information l_n−1 ^bby a predetermined parameter matrix.
Note that it is assumed that b=1, . . . , B and l₀ ^b=<sos> hold. Here, <sos> is a sentence head symbol.
Output Probability Distribution Calculation Unit 3
The intermediate feature H calculated by the intermediate feature calculation unit 1 and the character feature L_n−1 ^bcalculated by the character feature calculation unit 2 are input to the output probability distribution calculation unit 3.
The output probability distribution calculation unit 3 calculates, using the intermediate feature H and the character feature L_n−1 ^b, an output probability distribution Y_n ^bin which output probabilities corresponding to respective pieces of the first information are arranged (step S3).
The calculated output probability distribution Y_n ^bis output to the first information extraction unit 4.
The output probability distribution calculation unit 3 calculates an output probability distribution Y_n ^bin which the output probabilities corresponding to each unit of the output layer are arranged by inputting the intermediate feature H and the character feature L_n−1 ^bto an output layer of the predetermined neural network model. The output probability is, for example, a log probability. The output probability distribution is defined by Equation (2) of Reference 1, for example.
Assuming c=1, . . . , C for a given b, the processing from step S4 to step S7 described below is performed for each c. C is a predetermined positive integer. C may be an integer having the same value as B.
First Information Extraction Unit 4
The output probability distribution Y_n ^bcalculated by the output probability distribution calculation unit 3 is input to the first information extraction unit 4.
The first information extraction unit 4 extracts first information l_n ^{b, c}having a c-th highest probability of outputting in the output probability distribution Y_n ^b, and a score Score (l_n ^{b, c}), which is an output probability corresponding to the first information l_n ^{b, c}(step S4).
The extracted first information l_n ^{b, c}and score Score (l_n ^{b, c}) are output to the hypothesis creation unit 5.
Hypothesis Creation Unit 5
The first information l_n ^{b, c}and the score Score (l_n ^{b, c}) extracted by the first information extraction unit 4 are input to the hypothesis creation unit 5. Further, a first information sequence l_1:n−1 ^bup to the index n−1 that is previous one of the index n, selected by the hypothesis selection unit 8 and a score Score (l_1:n-−1 ^b) representing a likelihood of the first information sequence l_1:n−1 ^bare input to the hypothesis creation unit 5.
The hypothesis creation unit 5 creates a first information sequence l_1:n ^{b, c}in which the first information sequence l_1:n−1 ^band the first information l_n ^{b, c}are coupled, and the score Score (l_1:n ^{b, c}) representing a likelihood of the first information sequence l_1:n ^{b, c}(step S5).
The first information sequence l_1:n ^{b, c}is output to the first conversion unit 6 and the hypothesis selection unit 8. The score Score (l_1:n ^{b, c}) is output to the score integration unit 7.
The hypothesis creation unit 5 creates the score Score (l_1:n ^{b, c}) defined by, for example, the following equation.
Score (l_1:n ^{b, c})=Score (l_1:n−1 ^b)+Score (l_n ^{b, c})
First Conversion Unit 6
A first information sequence l_1:n ^{b, c}is input to the first conversion unit 6.
The first conversion unit 6 converts the first information sequence l_1:n ^{b, c}into a second information sequence w_1:o ^{b, c}using a predetermined model, and obtains a score Score (w_1:o ^{b, c}) representing a likelihood of the second information sequence w_1:o ^{b, c}(step S6).
The score Score (w_1:o ^{b, c}) is output to the score integration unit 7. o is a positive integer and is the number of pieces of second information.
As the predetermined model, for example, an attention-based model similar to sequence conversion of the acoustic feature⇒phonemic sequence can be used. Further, as the predetermined model, a statistical/neural transliteration model (for example, a model that converts a “kana sequence” which is the first information sequence into a “kana-kanji mixture sequence” which is the second information sequence) described in Reference 2 can be used. [Reference 2] L. Haizhou et. al, “A Joint Source-Channel Model for Machine Transliteration,” ACL, 2004
Score Integration Unit 7
The score Score (l_1:n ^{b, c}) created by the hypothesis creation unit 5 and the score Score (w_1:o ^{b, c}) obtained by the first conversion unit 6 are input to the score integration unit 7.
The score integration unit 7 obtains a new score Score (l_1:n ^{b, c}) that integrates a score Score (l_1:n ^{b, c}) and the score Score (w_1:n ^{b, c}) (step S7).
The obtained new score Score (l_1:n ^{b, c}) is output to the hypothesis selection unit 8.
For example, the score integration unit 7 obtains the new score Score (l_1:n ^{b, c}) defined by the following equation. Here, λ is a predetermined real number. For example, 0<λ<1.
Score (l_1:n ^{b, c})=Score (l_1:n ^{b, c})+λ·Score (w_1:o ^{b, c})
As described above, assuming B=1, . . . , B, processing from step S2 to step S7 described below is performed for each b. Further, assuming c=1, . . . , C, processing of step S4 to step S7 is performed for each c. Thus, assuming b=1, . . . , b and c=1, . . . , C, a new score Score (l_1:n ^{b, c}) corresponding to each of B×C sets (b, c) of b, c are obtained.
Hypothesis Selection Unit 8
The new score Score (l_1:n ^{b, c}) obtained by the score integration unit 7 is input to the hypothesis selection unit 8. Further, the first information sequence l_1:n ^{b, c}created by the hypothesis creation unit 5 is input to the hypothesis selection unit 8.
On the basis of the new score Score (l_1:n ^{b, c}), the hypothesis selection unit 8 selects B new scores including the high new score Score (l_1:n ^{b, c}). Then, the hypothesis selection unit 8 generates a new hypothesis including new scores selected and a first information sequence corresponding to the new score to set this new hypothesis to new hypotheses HypSet(1), . . . , HypSet(B) to be used at the index n+1 that is one after the index n that is currently being processed (step S8).
The generated new hypothesis HypSet(b) is output to the hypothesis creation unit 5 and to the second conversion unit 10. Further, the first information l_n ^bin the first information sequence l_1:n ^bincluded in the created hypothesis HypSet(b) is output to the character feature calculation unit 2.
Here, the first information sequence corresponding to the new score Score (l_1:n ^{b, c}) is the first information sequence l_i:n ^{b, c}.
A b-th new score having the high new score Score (l_1:n ^{b, c}) is expressed as the score Score (l_1:n ^b), and the first information sequence corresponding to the b-th new score having the high new score Score (l_1:n ^{b, c}) is expressed as the first information sequence l_1:n ^b. With these notations, when b=1, . . . , B holds, the new hypothesis HypSet(b) includes the score Score (l_1:n ^b) and the first information sequence l_1:n ^b. Accordingly, assuming b=1, . . . , B, the new hypothesis HypSet(b) can be expressed as the HypSet(b)=(l_1:n ^b, Score (l_1:n ^b)).
At the index n+1 that is one index after the index n that is currently being processed, the HypSet(b)=(l_1:n ^b, the Score (l_1:n ^b)) is HypSet(b)=(l_1:n−1 ^b, and the Score (l_1:n−1 ^b)), due to the fact that n is incremented by one. Thus, in FIG. 1 , the input of the hypothesis creation unit 5 is expressed as l_1:n−1 ^b, Score (l_1:n−1 ^b), and the input of the character feature calculation unit 2 is expressed as l_1:n−1 ^b.
Control Unit 9
The control unit 9 repeats the processing of the intermediate feature calculation unit 1, the character feature calculation unit 2, the output probability distribution calculation unit 3, the first information extraction unit 4, the hypothesis creation unit 5, the first conversion unit 6, the score integration unit 7, and the hypothesis selection unit 8 until a predetermined end condition is satisfied (step S9).
The predetermined end condition is n=N_MAX+1. N_MAXis the number of pieces of second information to be output, and is a predetermined positive integer. In this case, the control unit 9 increments n by one after processing of the hypothesis selection unit 8 ends. Then, the control unit 9 determines whether n=N_MAX+1 holds, and when n=N_MAX+1 holds, the control unit 9 ends the processing of the speech recognition apparatus. When n=N_MAX+1 does not hold, the control unit 9 performs control so as to return to the processing in step S2.
Further, the predetermined end condition may be l_n−1 ^b=<eos>. Here, <eos> is an end of sentence symbol.
Second Conversion Unit 10
The new hypotheses HypSet(1), . . . , HypSet(b) generated in the hypothesis selection unit 8 are input to the second conversion unit 10.
When the predetermined end condition is satisfied, the second conversion unit 10 converts at least a first information sequence l_1:n ¹corresponding to a score Score (l_1:n ¹) having a highest value into a second information sequence w_1:o ¹using a predetermined model (step S10).
The converted second information sequence w_1:o ¹is output from the speech recognition apparatus.
The prescribed model is, for example, the same model as the predetermined model of the first conversion unit 6.
In this manner, by taking into account the conversion processing of the “first information sequence⇒second information sequence” in the subsequent stage in the conversion processing of the “acoustic feature⇒first information sequence” in the previous stage, the present embodiment can achieve speech recognition with higher performance than that in the related art.
More specifically, in the present embodiment, the score integration unit 7 obtains the new score Score (l_1:n ^{b, c}) that integrates the score Score (l_1:n ^{b, c}) and the score Score (w_1:o ^{b, c}). This new score Score (l_1:n ^{b, c}) becomes the score Score (l_1:n ^b) in the hypothesis selection unit 8. Thus, the score Score (l_1:n ^b) can be said to take into account the score Score (w_1:o ^{b, c}). By extracting the first information on the basis of the score Score (l_1:n ^b) taking into account this score Score (w_1:o ^{b, c}), speech recognition with higher performance than in the related-art can be achieved.
Modified Examples
Although the embodiments of the present disclosure have been described above, it is obvious that a specific configuration is not limited to the embodiments, and the present disclosure also includes configurations appropriately changed in the design without departing from the gist of the present disclosure.
The various kinds of processing described in the embodiments are not only implemented in the described order in a time-series manner but may also be implemented in parallel or separately as necessary or in accordance with a processing capability of the apparatus which performs the processing.
For example, data exchange between components of the speech recognition apparatus may be performed directly, or may be performed via a storage unit that is not illustrated.
Program and Recording Medium
When various processing functions in each apparatus described above are implemented by a computer, processing content of the functions that each apparatus should have is described by a program. In addition, when the program is executed by the computer, the various processing functions of each device described above are implemented on the computer. For example, a variety of processing described above can be performed by causing a recording unit 2020 of the computer illustrated in FIG. 3 to read a program to be executed and causing a control unit 2010, an input unit 2030, an output unit 2040, and the like to execute the program.
The program in which the processing details are described can be recorded on a computer-readable recording medium. The computer-readable recording medium, for example, may be any type of medium such as a magnetic recording device, an optical disc, a magneto-optical recording medium, or a semiconductor memory.
In addition, the program is distributed, for example, by selling, transferring, or lending a portable recording medium such as a DVD or a CD-ROM with the program recorded on it. Further, the program may be stored in a storage device of a server computer and transmitted from the server computer to another computer via a network, so that the program is distributed.
For example, a computer executing the program first temporarily stores the program recorded on the portable recording medium or the program transmitted from the server computer in its own storage device. When executing the processing, the computer reads the program stored in its own storage device and executes the processing in accordance with the read program. Further, as another execution mode of this program, the computer may directly read the program from the portable recording medium and execute processing in accordance with the program, or, further, may sequentially execute the processing in accordance with the received program each time the program is transferred from the server computer to the computer. In addition, another configuration may be employed to execute the processing through a so-called application service provider (ASP) service in which processing functions are implemented just by issuing an instruction to execute the program and obtaining results without transmitting the program from the server computer to the computer. Note that the program in this embodiment includes information used for processing by a computer that is equivalent to the program (data or the like that has characteristics of regulating processing of the computer that is not a direct instruction to the computer).
In addition, although the device is configured by executing a predetermined program on a computer in this mode, at least a part of the processing details may be implemented by hardware.

REFERENCE SIGNS LIST

1 Intermediate feature calculation unit
2 Character feature calculation unit
3 Output probability distribution calculation unit
4 First information extraction unit
5 Hypothesis creation unit
6 First conversion unit
7 Score integration unit
8 Hypothesis selection unit
9 Control unit
10 Second conversion unit

Claims

1. A speech recognition apparatus in which B and C are predetermined positive integers, b=1, . . . , B and c=1, . . . , C hold, and a hypothesis HypSet(b) includes a first information sequence l_1:n−1 ^bfrom an index 1 to an index n−1 immediately before index n that is currently being processed, and a score Score (l_1:n−1 ^b) representing a likelihood of the first information sequence l_1:n−1 ^b, the speech recognition apparatus comprising a processor configured to execute a method comprising:

iteratively processing, until a predetermined end condition is satisfied, at least:

receiving an input acoustic feature in a predetermined neural network;

calculating an intermediate feature;

calculating a character feature L_n−1 ^bcorresponding to first information l_n−1 ^bof the index n−1 in a hypothesis b;

calculating, using the intermediate feature and the character feature L_n−1 ^b, an output probability distribution Y_n ^bin which a plurality of output probabilities corresponding to respective pieces of the first information are arranged;

extracting first information l_n ^{b, c}having a c-th highest output probability among the output probability distributions Y_n ^b, and a score Score (l_n ^{b, c}) that is an output probability corresponding to the first information l_n ^{b, c};

creating a first information sequence l_1:n ^{b, c}coupling the first information sequence l_1:n−1 ^band the first information l_n ^{b, c}, and a score Score (l_1:n ^{b, c}) representing a likelihood of the first information sequence l_1:n ^{b, c};

converting the first information sequence l_1:n ^{b, c}into a second information sequence w_1:o ^{b, c}using a predetermined model, and obtain

obtaining a score Score (w_1:o ^{b, c}) representing a likelihood of the second information sequence w_1:o ^{b, c};

obtaining a score integration unit configured to obtain a new score Score (l_1:n ^{b, c}) that integrates the score Score (l_1:n ^{b, c}) and the score Score (w_1:o ^{b, c});

selecting B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c});and

generating a new hypothesis including a plurality of new scores selected and a first information sequence corresponding to the plurality of new scores to set new hypotheses HypSet(1), . . . , HypSet(b) to be used at an index n+1 that is immediately after the index n that is currently being processed; and

when the predetermined end condition is satisfied, converting at least a first information sequence l_1:n ¹corresponding to a score Score (l_1:n ¹) having a highest value into a second information sequence w_1:0 ¹, using a predetermined model.

2. A speech recognition method in which B and C are predetermined positive integers, b=1, . . . , B and c=1, . . . , C hold, and a hypothesis HypSet(b) includes a first information sequence l_1:n−1 ^bfrom an index 1 to an index n−1 immediately before an index n that is currently being processed, and a score Score (l_1:n−1 ^b) representing a likelihood of the first information sequence l_1:n−1 ^b, the speech recognition method comprising:

iteratively processing, based on a predetermined condition, at least:

inputting an input acoustic feature in a predetermined neural network and calculating an intermediate feature;

creating a first information sequence l_1:n−1 ^b, c coupling the first information sequence _1:n ^{b, c}and the first information l_n ^{b, c}, and a score Score (l_1:n ^{b, c}) representing a likelihood of the first information sequence _1:n ^{b, c};

converting the first information sequence l_1:n ^{b, c}into a second information sequence w_1:o ^{b, c}using a predetermined model, and obtain a score Score (w_1:o ^{b, c}) representing a likelihood of the second information sequence w_1:o ^{b, c};

obtaining a new score Score (l_1:n ^{b, c}) that integrates the score Score (l_1:n ^{b, c}) and the score Score (w_1:o ^{b, c}); and

selecting B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c}), and generating a new hypothesis including a plurality of new scores selected and a first information sequence corresponding to the plurality of new scores to set new hypotheses HypSet(1), . . . , HypSet(b) to be used at index n+1 immediately after the index n that is currently being processed; and

when the predetermined end condition is satisfied, converting at least a first information sequence l_1:n ¹corresponding to a score Score (l_1:n ¹) having a highest value into a second information sequence w_1:o ¹, using a predetermined model.

3. A computer-readable non-transitory recording medium storing computer-executable program instructions that when executed by a processor cause a computer execute a speech recognition method comprising:

wherein B and C are predetermined positive integers, b=1, . . . , B and c=1, . . . , C hold, and a hypothesis HypSet(b) includes a first information sequence from an index 1 to an index n−1 immediately before an index n that is currently being processed, and a score Score (l_1:n−1 ^b) representing a likelihood of the first information sequence l_1:n−1 ^b,

iteratively processing, based on a predetermined condition, at least:

converting the first information sequence l_1:n ^{b, c}into a second information sequence w_1:o ^{b, c}using a predetermined model, and obtain a score Score (w_1:o ^{b, c}) representing a likelihood of the second information sequence w_1:o ^{b, c}.

4. The speech recognition apparatus according to claim 1, wherein the predetermined condition is based on a number of pieces of second information for output.

5. The speech recognition apparatus according to claim 1, wherein the predetermined condition is based on an end of sentence feature extracted from the first information.

6. The speech recognition apparatus according to claim 1, wherein the first information includes at least one of a phoneme of a grapheme associated with the input acoustic feature.

7. The speech recognition apparatus according to claim 1, wherein the second information includes a word including a symbol.

8. The speech recognition apparatus according to claim 1, wherein the first information is based on a first language, the second information is based on a second language, and the first language is distinct from the second language.

9. The speech recognition apparatus according to claim 1, wherein the selecting the B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c}) further includes causing an improvement in performing the extracting first information l_n ^{b, c}during a subsequent iteration of the iterative processing.

10. The speech recognition method according to claim 2, wherein the predetermined condition is based on a number of pieces of second information for output.

11. The speech recognition method according to claim 2, wherein the predetermined condition is based on an end of sentence feature extracted from the first information.

12. The speech recognition method according to claim 2, wherein the first information includes at least one of a phoneme of a grapheme associated with the input acoustic feature.

13. The speech recognition method according to claim 2, wherein the second information includes a word including a symbol.

14. The speech recognition method according to claim 2, wherein the first information is based on a first language, the second information is based on a second language, and the first language is distinct from the second language.

15. The speech recognition method according to claim 2, wherein the selecting the B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c}) further includes causing an improvement in performing the extracting first information l_n ^{b, c}during a subsequent iteration of the iterative processing.

16. The computer-readable non-transitory recording medium according to claim 3, wherein the predetermined condition is based on a number of pieces of second information for output.

17. The computer-readable non-transitory recording medium according to claim 3, wherein the predetermined condition is based on an end of sentence feature extracted from the first information.

18. The computer-readable non-transitory recording medium according to claim 3, wherein the first information includes at least one of a phoneme of a grapheme associated with the input acoustic feature, and wherein the second information includes a word including a symbol.

19. The computer-readable non-transitory recording medium according to claim 3, wherein the first information is based on a first language, the second information is based on a second language, and the first language is distinct from the second language.

20. The computer-readable non-transitory recording medium according to claim 3, wherein the selecting the B new scores having the high new score Score (l_1:n ^{b, c}) on a basis of the new score Score (l_1:n ^{b, c}) further comprises causing an improvement in performing the extracting first information l_n ^{b, c}during a subsequent iteration of the iterative processing.